A method and a system for determining the surface tension of a solution contained in a container are known, for example, from German Patent Application DE 100 22 863 A1.
The measurement of the dynamic surface tension according to the bubble pressure method is based on the dependence of the surface tension on the pressure during the formation of curved surfaces. It is known, for example, from German Patent Applications DE 41 12 417 A1 and DE 195 29 787 A1, to determine the detergent concentration of a detergent/water mixture (washing liquid, for example, in the soapy water container of a washing machine) by measuring the surface tension. In this context, the bubble pressure method has the advantage that the surface tension can be determined (dynamically) at surfaces of different age. The slower a bubble is formed, the more time the surfactants have to get to the surface and to reduce the surface tension. This velocity of the surfactants is also determined in this method (see DE 100 29 505 A1).
In the bubble pressure method, a continuously supplied airflow creates a bubble in the washing liquid at a capillary. The differential pressure between the forming and the detaching bubble is then proportional to the surface tension. In order to create the bubbles, a gaseous medium, generally air, is introduced through a narrow capillary tube into the washing liquid to be sensed. If this capillary tube is arranged directly in the soapy water container, then there is a risk for the capillary orifice to become clogged by dirt particles after some time. This problem is increased if the gas bubbles are created by a pump, and if this pump is switched off after the measuring process when the soapy water container is full. Then, liquid flows into the capillary and further into the hose to which the capillary is connected. If a deposit of lime or dirt forms in this hose and comes off later, the detached particles are blown by the pump into the capillary, clogging the capillary. In order to avoid this, German Patent Application DE 100 25 430 A1proposes that the capillary for measuring the surface tension in a washing machine be accommodated in a vessel which is located in the wall of the soapy water container and filled by scooping devices on the washing drum. In addition, the intention is for the vessel to be regularly cleaned by inflowing water. The imbalance during spinning is used to empty the vessel. In this context, it is a disadvantage that the emptying of the vessel by the imbalance cannot be carried out in a defined manner because the imbalance occurring during spinning is sometimes greater and sometimes less. In addition, it is necessary to attach a scooping device to the washing drum, which is not easy to accomplish for reasons of space.
From German Patent Application DE 100 22 863 A1, it is known for the capillary to open into a measuring chamber, in which a column of liquid that rises above the level of the outlet of the capillary can be produced by negative pressure. This ensures that the gas flow in the capillary is always in the direction of the liquid-filled measuring chamber. The intention of this is that the risk of clogging should be low; however, small amounts of liquid are nevertheless drawn into the capillary because of the capillary effect and due to negative pressure in the measuring chamber caused by cooling. Small amounts of liquid already lead to the formation of additional air/liquid interfaces inside the measuring chamber and interfere with the measurement. A cleaning operation (piercing, known from German Patent Application DE 195 29 787 A1) would now indeed allow the geometry of the orifice to be retained and would prevent it from being clogged by deposits. However, this cannot prevent ingress of liquid into the measuring chamber either.
German Patent Application DE 196 53 752 A1describes a method and device for cleansing process measuring cells containing capillaries, using a combination of ultrasound in conjunction with a flow of cleaning gas and liquid. A device of this type is too expensive for broad application in domestic appliances such as washing machines and dishwashers.